dnsop P. Wouters
Internet-Draft Red Hat
Intended status: Experimental February 18, 2016
Expires: August 21, 2016
Chain Query requests in DNSdraft-ietf-dnsop-edns-chain-query-07
Abstract
This document defines an EDNS0 extension that can be used by a
security-aware validating resolver configured to use a Forwarder to
send a single query, requesting a complete validation path along with
the regular query answer. The reduction in queries potentially
lowers the latency and reduces the need to send multiple queries at
once. This extension mandates the use of source IP verified
transport such as TCP or UDP with EDNS-COOKIE so it cannot be abused
in amplification attacks.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 21, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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Currently, applications send out many UDP requests concurrently.
This requires more resources on the DNS client with respect to state
(cpu, memory, battery) and bandwidth. There is also no guarantee
that the initial set of UDP questions will result in all the records
required for DNSSEC validation. More round trips could be required
depending on the resulting DNS answers. This especially affects
high-latency links.
This document specifies an EDNS0 extension that allows a validating
Resolver running as a Forwarder to open a TCP connection to another
Resolver and request a DNS chain answer using one DNS query/answer
pair. This reduces the number of round trips to two. If combined
with long lived TCP or [TCP-KEEPALIVE] there is only one round trip.
While the upstream Resolver still needs to perform all the individual
queries required for the complete answer, it usually has a much
bigger cache and does not experience significant slowdown from last-
mile latency.
This EDNS0 extension allows the Forwarder to indicate which part of
the DNS hierarchy it already contains in its cache. This reduces the
amount of data required to be transferred and reduces the work the
upstream Recursive Resolver has to perform.
This EDNS0 extension is only intended to be sent by Forwarders to
Recursive Resolvers. It MUST be ignored by Authoritative Servers.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Terminology
The DNS terminology used in this document is that of [RFC7719].
Additionally, the following terms are used:
Recursive Resolver: A nameserver that is responsible for resolving
domain names for clients by following the domain's delegation
chain, starting at the root. Recursive Resolvers frequently use
caches to be able to respond to client queries quickly. Described
in [RFC1035] chapter 7.
Validating Resolver: A recursive nameserver that also performs
DNSSEC [RFC4033] validation. Also known as "security-aware
resolver".
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Internet-Draft Chain Query requests in DNS February 20163. Overview
When DNSSEC is deployed on a host, it can no longer delegate all DNS
work to the upstream Recursive Resolver. Obtaining just the DNS
answer itself is not enough to validate that answer using DNSSEC.
For DNSSEC validation, the DNS client requires a locally running
validating Resolver so it can confirm DNSSEC validation of all
intermediary DNS answers. It can configure itself as a Forwarder if
it obtains the IP addresses of one or more Recursive Resolvers that
are available, or as a stand-alone Recursive Resolver if no
functional Recursive Resolvers were obtained. Generating the
required queries for validation adds a significant delay in answering
the DNS question of the locally running application. The application
must wait while the Resolver validates all intermediate answers.
Each round-trip adds to the total time waiting on DNS resolution with
validation to complete. This makes DNSSEC resolving impractical for
devices on networks with a high latency.
This document defines the CHAIN option that allows the Resolver to
request all intermediate DNS data it requires to resolve and validate
a particular DNS answer in a single round-trip. The Resolver could
be part of the application or a Recursive Resolver running on the
host.
Servers answering with CHAIN data should ensure that the transport is
TCP or source IP address verified UDP. See Section 8. This avoids
abuse in DNS amplification attacks.
Applications that support CHAIN internally can perform validation
without requiring the host to run a Recursive Resolver. This is
particularly useful for virtual servers in a cloud or container based
deployment where it is undesirable to run a Recursive Resolver per
virtual machine.
The format of this option is described in Section 4.
As described in Section 5.4, a Recursive Resolver could use this
EDNS0 option to include additional data required by the Resolver in
the Authority Section of the DNS answer packet when using a source IP
verified transport. The Answer Section remains unchanged from a
traditional DNS answer and contains the answer and related DNSSEC
entries.
An empty CHAIN EDNS0 option MAY be sent over any transport as a
discovery method. A DNS server receiving such an empty CHAIN option
SHOULD add an empty CHAIN option in its answer to indicate that it
supports CHAIN for source IP address verified transports.
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The mechanisms provided by CHAIN raise various security related
concerns, related to the additional work, bandwidth, amplification
attacks as well as privacy issues with the cache. These concerns are
described in Section 8.
4. Option Format
This draft uses an EDNS0 [RFC6891] option to include client IP
information in DNS messages. The option is structured as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-------------------------------+-------------------------------+
! OPTION-CODE ! OPTION-LENGTH !
+-------------------------------+-------------------------------+
~ Closest Trust Point (FQDN) ~
+---------------------------------------------------------------+
o OPTION-CODE, 2 octets, for CHAIN is 13.
o OPTION-LENGTH, 2 octets, contains the length of the payload
(everything after Option-length) in octets.
o Closest Trust Point, a variable length Fully Qualified Domain Name
("FQDN") in DNS wire format of the requested start point of the
chain. This entry is the 'lowest' known entry in the DNS chain
known by the recursive server seeking a CHAIN answer for which it
has a validated DS and DNSKEY record. The end point of the chain
is obtained from the DNS Query Section itself. No DNS name
compression is allowed for this value.
5. Protocol Description5.1. Discovery of Support
A Forwarder may include a zero-length CHAIN option in a regular query
over any transport to discover the DNS server capability for CHAIN.
Recursive Resolvers that support and are willing to accept CHAIN
queries over source IP verified transport respond to a zero-length
CHAIN received by including a zero-length CHAIN option in the answer.
If not already using a source IP verified transport, the Forwarder
MAY then switch to a source IP verified transport and start sending
queries with the CHAIN option to request a CHAIN response from the
Recursive Resolver. Examples of source IP verification are the 3-way
TCP handshake and UDP with [EDNS-COOKIE].
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Internet-Draft Chain Query requests in DNS February 20165.2. Generate a Query
In this option value, the Forwarder sets the Closest Trust Point in
the chain - furthest from the root - that it already has a DNSSEC
validated (secure or not) answer for in its cache. The upstream
Recursive Resolver does not need to include any part of the chain
from the root down to this option's FQDN. A complete example is
described in Section 9.1.
The CHAIN option should generally be sent by system Forwarders and
Resolvers within an application that also perform DNSSEC validation.
5.3. Send the Option
When CHAIN is available, the downstream Recursive Resolver can adjust
its query strategy based on the desired queries and its cache
contents.
A Forwarder can request the CHAIN option with every outgoing DNS
query. However, it is RECOMMENDED that Forwarders remember which
upstream Recursive Resolvers did not return the option (and
additional data) with their response. The Forwarder SHOULD fallback
to regular DNS for subsequent queries to those Recursive Resolvers.
It MAY switch to another Recursive Resolver that does support the
CHAIN option or try again later to see if the server has become less
loaded and is now willing to answer with Query Chains. A fallback
strategy similar to that described in [RFC6891] section 6.2.2 SHOULD
be employed to avoid persistent interference due to non-clean paths.
5.4. Generate a Response
When a query containing a non-zero CHAIN option is received from a
Forwarder, the upstream Recursive Resolver supporting CHAIN MAY
respond by confirming that it is returning a CHAIN. To do so, it
MUST set the CHAIN option to the lowest Trust Point sent as part of
the chain, with its corresponding OPTION-LENGTH. It extends the
Authority Section in the DNS answer packet with the DNS RRsets
required for validating the answer. The DNS RRsets added start with
the first chain element below the received Closest Trust Point up to
and including the NS and DS RRsets that represent the zone cut
(authoritative servers) of the QNAME. The added RRsets MAY be added
in matching hierarchical order but a DNS client MUST NOT depend on
the order of the added RRsets for validation. The actual DNS answer
to the question in the Query Section is placed in the DNS Answer
Section identical to the traditional DNS answer. All required DNSSEC
related records must be added to their appropriate sections. This
includes records required for proof of non-existence of regular and/
or wildcard records, such as NSEC or NSEC3 records.
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Recursive Resolvers that have not implemented or enabled support for
the CHAIN option, or are otherwise unwilling to perform the
additional work for a Chain Query due to work load, may safely ignore
the option in the incoming queries. Such a server MUST NOT include
an CHAIN option when sending DNS answer replies back, thus indicating
it is not able or willing to support Chain Queries at this time.
Requests with wrongly formatted options (i.e. bogus FQDN) MUST be
rejected and a FORMERR response must be returned to the sender, as
described by [RFC6891].
Requests resulting in chains that the receiving resolver is unwilling
to serve can be rejected by answering the query as a regular DNS
reply but with an empty CHAIN payload. Replying with an empty CHAIN
can be used for chains that would be too big or chains that would
reveal too much information considered private.
At any time, a Recursive Resolver that has determined that it is
running low on resources can refuse CHAIN queries by replying with a
regular DNS reply with an empty CHAIN payload.
If a CHAIN answer would be bigger than the Recursive Resolver is
willing to serve, it SHOULD send a partial chain starting with the
data closest to the top of the chain. The client MAY re-send the
query with an updated Closest Trust Point until it has received the
full chain. The CHAIN response will contain the lowest Closest Trust
Point that was included in the CHAIN answer.
If the DNS request results in an CNAME or DNAME for the Answer
Section, the Recursive Resolver MUST return these records in the
Answer Section similar to regular DNS processing. The CNAME or DNAME
target MAY be placed in the Additional Section only if all supporting
records for DNSSEC validation of the CNAME or DNAME target are also
added to the Authority Section.
The response from a Recursive Resolver to a Resolver MUST NOT contain
the CHAIN option if none was present in the Resolver's original
request.
A DNS query that contains the CHAIN option MUST also have the DNSSEC
OK ("OK") bit set. If this bit is not set, or if the Checking
Disabled ("CD") bit is set, the CHAIN option received MUST be
ignored.
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The presence or absence of an OPT resource record containing an CHAIN
option in a DNS query does not change the usage of those resource
records and mechanisms used to provide data origin authentication and
data integrity to the DNS, as described in [RFC4033], [RFC4034] and
[RFC4035].
6.2. NS record Considerations
CHAIN responses SHOULD include the NS RRset from the zone itself
including the RRSIG records required for validation. It MUST NOT
include the NS RRset from parent zone, as this RRset is not signed.
If the size of the answer is an important factor, the NS RRset MAY be
omited.
When a DNSSEC chain is supplied via CHAIN, the Forwarder is no longer
required to use the NS RRset, as it can construct the validation path
via the DNSKEY and DS RRsets without using the NS RRset. However,
the Forwarder might be forced to switch from Forwarder mode to
Recursive Resolver mode due to a network topology change. In
Recursive Resolver mode, the NS RRsets are needed to find and query
Authoritative Servers directly. It is RECOMMENDED that the DNS
Forwarder populate its cache with this information to avoid requiring
future queries to obtain any missing NS records. Therefore, CHAIN
responses MUST include the NS RRset from the child zone, including
the RRSIG records required for validation.
6.3. Session Management
The use of [TCP-KEEPALIVE] on DNS TCP sessions is RECOMMENDED, and
thus TCP sessions should not immediately be closed after the DNS
answer to the first query is received.
Both DNS clients and servers are subject to resource constraints
which will limit the extent to which Chain Queries can be executed.
Effective limits for the number of active sessions that can be
maintained on individual clients and servers should be established,
either as configuration options or by interrogation of process limits
imposed by the operating system.
In the event that there is greater demand for Chain Queries than can
be accommodated, DNS servers may stop advertising the CHAIN option in
successive DNS messages. This allows, for example, clients with
other candidate servers to query to establish new sessions with
different servers in expectation that those servers might still allow
Chain Queries.
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If a CHAIN answer would intersect with a Negative Trust Anchor
[RFC7646], a partial CHAIN up to the node above the Negative Trust
Anchor should be returned.
6.5. Anycast Considerations
Recursive Resolvers of various types are commonly deployed using
anycast [RFC4786].
Successive DNS transactions between a client and server using UDP
transport may involve responses generated by different anycast nodes,
and the use of anycast in the implementation of a DNS server is
effectively undetectable by the client. The CHAIN option SHOULD NOT
be included in responses using UDP transport from servers provisioned
using anycast unless all anycast server nodes are capable of
processing the CHAIN option.
Since DNS queries using CHAIN may result in longer TCP sessions,
network topology changes may disrupt them more frequently. Anycast
servers MAY make use of TCP multipath [RFC6824] to anchor the server
side of the TCP connection to an unambiguously-unicast address in
order to avoid disruption due to topology changes.
7. Implementation Status
[RFC Editor Note: Please remove this entire seciton prior to
publication as an RFC.]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC6982], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
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[While there is some interest, no work has started yet]
8. Security Considerations8.1. Additional work and bandwidth
Producing CHAIN answers incurs additional load and bandwidth on the
Recursive Resolver. At any time, a Recursive Resolver may decide to
no longer answer with CHAIN answers and fall back to traditional DNS
answers.
8.2. Amplification Attacks
Chain Queries can potentially send very large DNS answers. Attackers
could abuse this using spoofed source IP addresses to inflict large
Distributed Denial of Service attacks using query-chains as an
amplification vector in their attack. While TCP is not vulnerable
for this type of abuse, the UDP protocol is vulnerable to this.
A Recursive Resolver MUST NOT return CHAIN answers to clients over
UDP without source IP address verification. An example of UDP based
source IP address verification is [EDNS-COOKIE]. A Recursive
Resolver refusing a CHAIN option MUST respond with a zero-length
CHAIN option to indicate support for CHAIN queries when a proper
transport is used. It MUST NOT send an RCODE of REFUSED.
8.3. Privacy Considerations
A client producing CHAIN queries reveals a little more information
about its cache contents than regular DNS clients. This could be
used the fingerprint a client across network reconnections. If DNS
privacy is a concern, a CHAIN query client MAY try to use a DNS
transport that provides privacy, such as [DNS-over-TLS] or a trusted
DNS server that is contacted through a VPN connection such as IPsec.
9. Examples9.1. Simple Query for example.com
o A web browser on a client machine asks the Forwarder running on
localhost to resolve the A record of "www.example.com." by sending
a regular DNS UDP query on port 53 to 127.0.0.1.
o The Resolver on the client machine checks its cache, and notices
it already has a DNSSEC validated entry of "com." in its cache.
This includes the DNSKEY RRset with its RRSIG records. In other
words, according to its cache, ".com" is DNSSEC validated as
"secure" and can be used to continue a DNSSEC validated chain.
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o The Resolver on the client opens a TCP connection to its upstream
Recursive Resolver on port 53. It adds the CHAIN option as
follows:
* Option-code, set to 13
* Option-length, set to 5
* Closest Trust Point set to "com." (0x03 0x63 0x6f 0x6d 0x00)
o The upstream Recursive Resolver receives a DNS query over TCP with
the CHAIN Closest Trust Point set to "com.". After accepting the
query it starts constructing a DNS reply packet.
o The upstream Recursive Resolver performs all the regular work to
ensure it has all the answers to the query for the A record of
"www.example.com.". It does so without using the CHAIN option -
unless it is also configured as a Forwarder. The answer to the
original DNS question could be the actual A record, the DNSSEC
proof of non-existence, or an insecure NXDOMAIN response.
o The upstream Recursive Resolver adds the CHAIN option to the DNS
response as follows:
* Option-code, set to 13
* Option-length, set to 5
* The Closest Trust Point is set to "com." (0x03 0x63 0x6f 0x6d
0x00)
o The upstream Recursive Resolver constructs the DNS Authority
Section and fills it (in any order) with:
* The DS RRset for "example.com." and its corresponding RRSIGs
(made by the "com." DNSKEY(s))
* The DNSKEY RRset for "example.com." and its corresponding
RRSIGs (made by the "example.com" DNSKEY(s))
* The authoritative NS RRset for "example.com." and its
corresponding RRSIGs (from the child zone)
If the answer does not exist, and the zone uses DNSSEC, it also
adds the proof of non-existence, such as NSEC or NSEC3 records, to
the Authority Section.
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o The upstream Recursive Resolver constructs the DNS Answer
Section and fills it with:
* The A record of "www.example.com." and its corresponding RRSIGs
If the answer does not exist (NODATA or NXDOMAIN), the Answer
Section remains empty. For the NXDOMAIN case, the RCode of the
DNS answer packet is set to NXDOMAIN. Otherwise it remains
NOERROR.
o The upstream Recursive Resolver returns the DNS answer over the
existing TCP connection. When all data is sent, it SHOULD keep
the TCP connection open to allow for additional incoming DNS
queries - provided it has enough resources to do so.
o The Resolver on the client receives the DNS answer. It processes
the Authority Section and the Answer Section and places the
information in its local cache. It ensures that no data is
accepted into the cache without having proper DNSSEC validation.
It MAY do so by looping over the entries in the Authority and
Answer Sections. When an entry is validated for its cache, it is
removed from the processing list. If an entry cannot be validated
it is left in the process list. When the end of the list is
reached, the list is processed again until either all entries are
placed in the cache, or the remaining items cannot be placed in
the cache due to lack of validation. Those entries are then
discarded.
o If the cache contains a valid answer to the application's query,
this answer is returned to the application via a regular DNS
answer packet. This packet MUST NOT contain an CHAIN option. If
no valid answer can be returned, normal error processing is done.
For example, an NXDOMAIN or an empty Answer Section could be
returned depending on the error condition.
9.2. Out-of-path Query for example.com
A Recursive Resolver receives a query for the A record for
example.com. It includes the CHAIN option with the following
parameters:
o Option-code, set to 13
o Option-length, set to 14
o The Closest Trust Point set to 'unrelated.ca.' (0x09 0x75 0x6e
0x72 0x65 0x6c 0x61 0x74 0x65 0x64 0x03 0x63 0x61 0x00)
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As there is no chain that leads from "unrelated.ca." to
"example.com", the Resolving Nameserver answers with an empty CHAIN
specified using:
o Option-code, set to 13
o Option-length, set to 0x00 0x00
o The Closest Trust Point is omitted (zero length)
Note that the regular answer is still present just as it would be for
a query that did not specify the CHAIN option.
9.3. Non-existent data
A Recursive Resolver receives a query for the A record for
"ipv6.toronto.redhat.ca". It includes the CHAIN option with the
following parameters:
o Option-code, set to 13
o Option-length, set to 0x00 0x03
o The Closest Trust Point set to 'ca.'
Using regular UDP queries towards Authoritative Nameservers, it
locates the NS RRset for "toronto.redhat.ca.". When querying for the
A record it receives a reply with RCODE "NoError" and an empty Answer
Section. The Authority Section contains NSEC3 and RRSIG records
proving there is no A RRtype for the QNAME "ipv6.toronto.redhat.ca".
The Recursive Resolver constructs a DNS reply with the following
CHAIN option parameters:
o Option-code, set to 13
o Option-length, set to 0x00 0x00
o The Closest Trust Point is ommited (zero length)
The RCODE is set to "NoError". The Authority Section is filled in
with:
o The DS RRset for "redhat.ca." plus RRSIGs
o The DNSKEY RRset for "redhat.ca." plus RRSIGs
o The NS RRset for "redhat.ca." plus RRSIGs (eg ns[01].redhat.ca)
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o The A RRset for "ns0.redhat.ca." and "ns1.redhat.ca." plus RRSIGs
o The DS RRset for "toronto.redhat.ca." plus RRSIGs
o The NS RRset for "toronto.redhat.ca." plus RRSIGs (eg
ns[01].toronto.redhat.ca)
o The DNSKEY RRset for "toronto.redhat.ca." plus RRSIGs
o The A RRset and/or AAAA RRset for "ns0.toronto.redhat.ca." and
"ns1.toronto.redhat.ca." plus RRSIGs
o The NSEC record for "ipv6.toronto.redhat.ca." (proves what RRTYPEs
do exist, does not include A)
o The NSEC record for "toronto.redhat.ca." (proves no wildcard
exists)
The Answer Section is empty. The RCode is set to NOERROR.
10. IANA Considerations10.1. EDNS0 option code for CHAIN
IANA has assigned option code 13 in the "DNS EDNS0 Option Codes
(OPT)" registry to CHAIN.
11. Acknowledgements
Andrew Sullivan pointed out that we do not need any new data formats
to support DNS chains. Olafur Gudmundsson ensured the RRsets are
returned in the proper Sections. Thanks to Tim Wicinski for his
thorough review.
12. Normative References
[DNS-over-TLS]
Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "DNS over TLS: Initiation and Performance
Considerations", draft-ietf-dprive-dns-over-tls-05 (work
in progress), January 2016.
[EDNS-COOKIE]
Eastlake, Donald., "Domain Name System (DNS) Cookies",
draft-ietf-dnsop-cookies (work in progress), January 2016.
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